Time-dependent mobility and recombination of the photoinduced charge carriers in conjugated polymer/fullerene bulk heterojunction solar cells

Mozer, Attila J.; Dennler, Gilles; Sariciftci, Niyazi Serdar; Westerling, M.; Pivrikas, Almantas; Österbacka, Ronald; Juška, G.

doi:10.1103/physrevb.72.035217

Cited by 217 publications

(216 citation statements)

References 48 publications

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“…The photogenerated carriers were then extracted by the triangular voltage pulse in reverse bias. 14 The faster carrier mobility can be calculated from photo-CELIV to be…”

Section: Resultsmentioning

confidence: 99%

On the unipolarity of charge transport in methanofullerene diodes

et al. 2017

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Fullerenes are electron transporting organic semiconductors with a wide range of applications. In particular, methanofullerenes have been the preferred choice for solution-processed solar cells and photodiodes. The wide applicability of fullerenes as both ‘n-type’ transport materials and electron acceptors is clear. However, what is still a matter of debate is whether the fullerenes can also support efficient transport of holes, particularly in diode geometries. In this letter, we utilize a number of recently developed experimental methods for selective electron and hole mobility measurements. We show for the two most widely used solution processable fullerenes, PC70- and-PC60BM, that whilst both exhibit electron mobilities as high as 10−3 cm2/Vs, their hole mobilities are < 10−9 cm2/Vs. Thus charge transport in these fullerenes can be considered predominantly unipolar in diode configurations.

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“…The photogenerated carriers were then extracted by the triangular voltage pulse in reverse bias. 14 The faster carrier mobility can be calculated from photo-CELIV to be…”

Section: Resultsmentioning

confidence: 99%

On the unipolarity of charge transport in methanofullerene diodes

et al. 2017

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“…Unfortunately, as researchers seek to increase the efficiency of polymer photovoltaics and companies begin the process of upscaling this technology, increasing the thickness of the active layer of the BHJ becomes necessary due to the minimum film thicknesses that can be reproducibly fabricated from scalable printing processes (roll-to-roll printing, etc.). [6,69] Charge recombination has been studied via a variety of experimental methods, including time-of-flight (TOF), [70] steady-state current-voltage, [71][72][73][74] impedance spectroscopy, [75][76][77] photo induced charge extraction by linearly increasing voltage (photo-CELIV), [78] double injection currents, [79] transient absorption, and transient photovoltage measurements. [80,81] At applied voltages above the maximum power point and at the open circuit condition, bimolecular recombination reduces the current density and limits the fill factor, thereby decreasing of PCBM revealed that exciton hopping before charge separation only occurs in films with very low fullerene concentration and poor morphology, but does not play a role for favorable blends having ultrafast quantitative charge separation within 200 fs.…”

Section: Photoexcitation and Charge Transfermentioning

confidence: 99%

Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

Cowan

Banerji

Leong

et al. 2012

Adv Funct Materials

312

252

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This article presents a critical discussion of the various physical processes occurring in organic bulk heterojunction (BHJ) solar cells based on recent experimental results. The investigations span from photoexcitation to charge separation, recombination, and sweep‐out to the electrodes. Exciton formation and relaxation in poly[N‐9″‐hepta‐decanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and poly‐3(hexylthiophene) (P3HT) are discussed based on a fluorescence up‐conversion study. The commonly accepted paradigm describing the conversion of incident photons into charge carriers in the BHJ material is re‐examined in light of these femtosecond time‐resolved measurements. Transient photoconductivity, time‐delayed collection field, and time‐delayed dual pulse experiments carried out on BHJ solar cells demonstrate the competition between carrier sweep‐out by the internal field and the loss of photogenerated carriers by recombination. Finally, an emerging hypothesis is discussed: that bimolecular recombination accounts for the majority of recombination from short circuit to open circuit in optimized solar cells, and that bimolecular recombination is bias‐ and charge‐density‐dependent. The study of recombination loss processes in organic solar cells leads to insights into what must be accomplished to achieve the “ideal” solar cell.

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“…However, in other seemingly similar systems, recombination rates much closer to the Langevin model were observed. 14,26,27,[34][35][36] Even blends with very similar morphology can have dramatically different recombination rates.…”

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confidence: 99%

Charge carrier concentration dependence of encounter-limited bimolecular recombination in phase-separated organic semiconductor blends

2016

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Understanding how the complex intermolecular-and nano-structure present in organic semiconductor donor-acceptor blends impacts charge carrier motion, interactions, and recombination behavior is a critical fundamental issue with a particularly major impact on organic photovoltaic applications. In this study, kinetic Monte Carlo (KMC) simulations are used to numerically quantify the complex bimolecular charge carrier recombination behavior in idealized phase-separated blends. Recent KMC simulations have identified how the encounter-limited bimolecular recombination rate in these blends deviates from the often used Langevin model and have been used to construct the new power mean mobility model. Here, we make a challenging but crucial expansion to this work by determining the charge carrier concentration dependence of the encounter-limited bimolecular recombination coefficient. In doing so, we find that an accurate treatment of the long-range electrostatic interactions between charge carriers is critical, and we further argue that many previous KMC simulation studies have used a Coulomb cutoff radius that is too small, which causes a significant overestimation of the recombination rate. To shed more light on this issue, we determine the minimum cutoff radius required to reach an accuracy of less than ±10% as a function of the domain size and the charge carrier concentration and then use this knowledge to accurately quantify the charge carrier concentration dependence of the recombination rate. Using these rigorous methods, we finally show that the parameters of the power mean mobility model are determined by a newly identified dimensionless ratio of the domain size to the average charge carrier separation distance.

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Time-dependent mobility and recombination of the photoinduced charge carriers in conjugated polymer/fullerene bulk heterojunction solar cells

Cited by 217 publications

References 48 publications

On the unipolarity of charge transport in methanofullerene diodes

On the unipolarity of charge transport in methanofullerene diodes

Charge Formation, Recombination, and Sweep‐Out Dynamics in Organic Solar Cells

Charge carrier concentration dependence of encounter-limited bimolecular recombination in phase-separated organic semiconductor blends

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